FIG. 17 is a circuit diagram illustrating a transmitter of a conventional radio communication apparatus. In FIG. 17, a voltage controlled oscillator 17 receives an input signal S1 and outputs a signal S11 having a frequency in proportion to the voltage. A frequency synthesizer 16 generates a signal S12 for converting the signal S11 into a signal having a frequency to be transmitted. The signal S11 from the voltage controlled oscillator 17 and the signal S12 from the frequency synthesizer are input to a mixer 15. The mixer 15 compounds these signals and outputs a signal S13 to a voltage amplifier 14. The voltage amplifier 14 amplifies the signal S13 and outputs a signal S14. The signal S14 passes through a filter 13. The filter 13 eliminates a signal component of having an unnecessary frequency from the signal S14 and outputs a signal S14a. The signal S14a is radiated into space from an antenna 11 as a signal S2. A transmit-receive switch (hereinafter referred to as TR switch) 12 is interposed between the filter 13 and the antenna 11.
In radio communication systems, communication apparatus of different kinds have different frequency bands for communication (hereinafter referred to as usable frequency bands). If an automobile telephone having a usable frequency band ranging from 900 MHz to 920 MHz transmits signals having frequencies outside this range, these signals cause noise on a radio communication apparatus having a usable frequency band including the frequencies.
On the other hand, in radio communication using the same radio communication apparatus, each person uses his own frequency range in the usable frequency band of the communication apparatus. For example, when a radio communication apparatus has a usable frequency band from 900 MHz to 920 MHz, if the frequency range for each person, i.e., each communication, is 100 KHz, two hundred and one radio communications are possible.
FIG. 18 illustrates input frequency vs. output voltage characteristics of the filter 13 of the radio transmitter 800 shown in FIG. 17. The usable frequency band of this radio transmitter ranges from 900 MHz to 920 MHz. As shown in FIG. 18, the filter 13 has a pass band for suppressing signals of frequencies that are not included in the usable frequency band (900 MHz.about.920 MHz) so that the radio transmitter does not cause noise in other kinds of radio communication apparatus. For example, in the radio transmitter 800 shown in FIG. 17, if a signal having a frequency of 700 MHz is output from the frequency synthesizer 16 and a signal having a frequency of 210 MHz is output from the voltage controlled oscillator 17, a signal having a frequency of 910 MHz (700 MHz+210 MHz) and a signal with a frequency of 1120 MHz (700 MHz+210 MHz.times.2) are output from the voltage amplifier 14. Since the signal having a frequency of 1120 MHz is eliminated by the filter 13, it is not radiated into space.
As described above, in the prior art radio communication apparatus, signals produced in the apparatus and having frequencies other than the usable frequency band of the apparatus are not radiated into space. However, the transmission signal generating means comprising the voltage controlled oscillator 17, the frequency synthesizer 16, and the mixer 15 generate weak signals, i.e., signals at low amplitude levels, having frequencies that are within the usable frequency band but are different from a substantial communication frequency. These signals are not eliminated by the filter 13. For example, in a communication apparatus having a usable frequency band of 900.about.920 MHz and operating at a frequency of 911 MHz, if signals having a frequency of 912 MHz are produced by the transmission signal generating means, these signals are transmitted together with signals of 911 MHz and cause noise of in other communications using the frequency of 912 MHz.